1. Field of the Invention
The invention relates to a discharge lamp which is used for a liquid crystal display device and for a projector device, such as a DLP (digital light processor) or the like using a DMD (digital micro-mirror device). The invention relates especially to an ultra-high pressure mercury lamp in which the arc tube is filled with at least 0.15 mg/mm3 of mercury and in which the mercury vapor pressure during operation is at least 150 atm.
2. Description of the Prior Art
In a projector device of the projection type which includes a liquid crystal projector, DLP or the like using a DMD, there is a need for uniform illumination of images onto a rectangular screen, moreover with adequate color rendering. Therefore, the light source is a metal halide lamp filled with mercury and a metal halide. Recently, these metal halide lamps have been becoming smaller and smaller, and more and more often point light sources are being produced. In practice, they are used with extremely small distances between the electrodes.
Against this background, lamps with an extremely high mercury vapor pressure in operation, for example, with at least 150 atm, have recently been suggested instead of metal halide lamps. This increase of the mercury vapor pressure is designed to reduce broadening of the arc (the arc is compressed), and at the same time, to greatly increase the light intensity. Such an ultra-high pressure mercury lamp is disclosed, for example, in Japanese patent application publication JP-A-HEI 2-148561 which corresponds to U.S. Pat. No. 5,109,181 and in Japanese patent application JP-A-HEI 6-52830 which corresponds to U.S. Pat. No. 5,497,049.
Such an ultra-high pressure mercury lamp often has the disadvantage that blackening of the inside wall of the arc tube reduces the light transmittance. This is a result of the high operating temperature of the electrodes and frequent spraying of the tungsten electrode material during operation causing tungsten to be deposited on the inside wall of the bulb.
Therefore, the above described publications describe that the arc tube, which has a spherical light emitting part in its middle area, is filled with a given amount of halogen gas, and that blackening in the arc tube is prevented by carrying out the halogen cycle.
On the other hand, in the above described ultra-high pressure mercury lamp, there are also many cases in which the tungsten comprising the electrodes sprays during start-up. It can be imagined that this is caused by the following.
Normal operation of the ultra-high pressure mercury lamp takes place such that, at the start of operation, a glow discharge is formed in which the cathode constitutes the starting point, that the discharge starting point passes to the cathode tip when a high temperature state of the electrodes is reached by a glow discharge, and that it passes into rated operation due to the hot arc discharge.
However, since in this glow discharge the starting voltage (the voltage necessary for starting) is high, the tungsten sprays easily due to the fact that vigorous ion sputtering and thermal shocks are applied to the electrodes. This spraying of the tungsten leads directly to blackening of the arc tube, since the halogen cycle is inactive during starting. Therefore, spraying of the tungsten is regarded as more disadvantageous in start-up than in operation.